Production of proteins of interest is commonly achieved in transformed competent host cells. A problem that arises during purification of such proteins is that contaminant host proteins co-purify with the protein of interest. One approach to tackling this problem is to form a fusion protein between the protein of interest and a protein tag that has an affinity to a matrix. It is intended that the contaminant proteins are washed away and a pure protein is recovered. An example of a protein tag that is widely used is a histidine tag (His-tag). This binds to a metal containing column. The method is called immobilized metal ion affinity chromatography (see for example U.S. Pat. No. 5,310,663).
Unfortunately, contaminating host cell proteins which do not carry any form of tag may contain non-consecutive histidine residues or other metal binding motifs exposed to the surface of their ternary structure. These contaminating proteins also bind to nickel and/or cobalt containing purification resins to which the His-tagged protein of interest binds (see Bolanos-Garcia and Davies, BBA 1760: 1304-1313(2006), and Edwards, et al., Nature Methods 5: 135-146(2008)), resulting in co-purification of these contaminants and failure to obtain a purified preparation of the protein of interest.